As a method of measuring a flow volume of gas flowing in a flow channel, various methods have been proposed. For example, Patent Literature 1 describes a differential pressure flowmeter in which an orifice plate is arranged in a pipe, and a flow volume of fluid flowing in the pipe is measured by a pressure difference in the pipe at the front and back of the orifice plate.
Furthermore, Patent Literature 2 describes a flow volume measurement device in which a transmission element and a reception element of an ultrasonic wave are installed relative to each other with a certain angle in a flow direction of fluid, thereby measuring a flow volume of the fluid based on an ultrasonic propagation time. This flow volume measurement device includes means for oscillating an ultrasonic wave of a sinusoidal signal having frequency components of a first frequency and a second frequency that are different from each other, means for obtaining a phase difference between the first and second frequency components of the ultrasonic wave having propagated in the fluid, and means for computing an ultrasonic propagation time from the obtained phase difference. The flow volume measurement device calculates a flow velocity of the fluid from the ultrasonic propagation time to obtain the flow volume.